A two-level system ensures discussion and also keeps it easy to follow. You make a post, and others make replies below it to discuss about it. New replies are added at the end, and you can easily follow the discussion without having to follow any tangents, branches or spin-offs. This avoids the headaches of trees while giving readers the opportunity to contribute.
* unfortunately Facebook is starting to introduce "replies to comments" in some areas of the website (I think Pages), and that's kinda freaking me out. But still, three-levels should be manageable.
http://verchini.com/bop-da

It's been six years since I wrote Discussions: Flat or Threaded? and, despite a bunch of evolution on the web since then, my opinion on this has not fundamentally changed. If anything, my opinion has strengthened based on the observed data: precious few threaded discussion models survive on t...